The present invention relates to a method of making a blown vessel provided with a mouth, to an insert for being overmolded by blowing a parison, and to a blown vessel including such an overmolded insert.
The present invention relates more particularly to the field of fuel tanks made in the form of blown vessels of plastics material.
A fuel tank made of plastics material is provided with a hydrocarbon barrier which prevents or limits hydrocarbon emissions through its wall, e.g. in the form of leaks or by diffusion.
Known barriers are obtained by fluorination methods or they are constituted by a layer of ethylene vinyl alcohol copolymer (EVOH) inserted between two layers of polyethylene.
Vessels made in this way generally give satisfaction in terms of the hydrocarbon emission criterion.
Nevertheless, they require various arrangements, in particular to enable them to be connected to ducts, e.g. for delivering or receiving liquid fuel, and also for allowing gas to flow in the fuel system.
It is therefore necessary for such vessels to be pierced in order to put ducts into communication with the inside thereof and to make assemblies by screwing, heat-sealing, snap-fastening, or other means in order to secure said ducts to the wall of the vessel.
Such piercing and assembly operations give rise to passages through the hydrocarbon barriers and they thus make hydrocarbon emissions possible. French patent FR 2 459 121 describes a hollow body made by blowing, e.g. a motor vehicle tank, the hollow body including a piece of metal held captive in its wall, the captive piece having perforations into which a single layer material forming the hollow body penetrates while it is being blown, the captive piece being designed to co-operate with a stopper element.
The captive piece is thus retained very strongly in the wall of the hollow body.
German patent application DE-A-4 205 332 describes assembling a metal coupling element to a fuel tank made by blowing a single layer of material, in which the parison for forming the tank partially covers said coupling element.
Such an assembly serves to limit hydrocarbon emissions to some extent.
The present invention seeks to limit hydrocarbon emissions more effectively, or indeed to eliminate them.
The invention provides firstly a method of making a blown vessel provided with a passage suitable for putting the inside of the vessel into communication with the outside thereof and including a mouth on the outside, said method comprising the following steps:
overmolding an insert of tubular shape in a parison while it is being blown so that the parison covers one of the ends of the insert, while leaving the opposite end of the insert projecting outside the vessel so as to constitute the outside mouth of the passage, the vessel made from the parison having at least a first layer forming a hydrocarbon barrier and a second layer that is more permeable to hydrocarbons than the first and that is outside it; and
then eliminating at least a portion of the wall lying inside the insert, the insert being shaped to lengthen the diffusion path for hydrocarbons diffusing along the second layer from the edge of the vessel adjacent to the removed wall portion towards the outside of the vessel, and/or to form one or more regions of reduced thickness in said second layer along the diffusion path.
Since the vessel and the insert are secured to each other while the parison is being blown, the connection between them takes place without any interruption in the continuity of the wall of the vessel.
The second layer which is more permeable to hydrocarbons forms a preferred passage towards the outside for hydrocarbons. The reductions in the thickness of this second layer on the diffusion path and/or the lengthening of this path constitute corresponding obstacles to diffusion and make it possible to limit hydrocarbon emissions effectively.
The second layer may present a single region of reduced thickness, however it preferably presents at least two regions of reduced thickness, and better still three such regions.
The barrier-forming first layer can be in the form of an EVOH barrier layer and it can be disposed, for example, between inner and outer layers of the vessel, said barrier being neither cut nor pierced at the moment the insert is put into place.
The same applies to any other hydrocarbon barrier obtained, for example, by fluorination or by coating.
The sole disturbance to which the wall of the vessel is subject is deformation enabling it to fit closely around the shape of the end of the insert which it covers, which deformation is favorable to establishing a more effective barrier to hydrocarbons at the junction between the vessel and the insert because the barrier-forming layer which has been stretched by the deformation, presses more intimately against or comes very close to the insert, thus reducing the size of the gap available between the insert and said barrier for the passage of hydrocarbons.
The vessel can include at least one layer of polyethylene.
In a particular implementation of the invention, an insert is used that is made of a material that is poorly permeable to hydrocarbons, which material is referred to in the description below as a xe2x80x9clow permeability materialxe2x80x9d.
In yet another particular implementation of the invention, an adhesive is interposed between the parison and the end of the insert covered by said parison, thereby improving contact between the end of the insert covered by the wall and said wall, by providing adhesion.
The adhesive can be deposited on a localized area of the parison before the insert is overmolded, or on at least a portion of the end of the insert itself.
In a particular implementation of the invention, one or more peripheral grooves are formed in the end of the insert that is covered by the parison, with the parison penetrating into the grooves at the moment overmolding occurs during blowing. These grooves serve in particular to retain the insert mechanically on the wall of the vessel.
The peripheral-groove(s) can include an inner groove opening out into the inside space of the insert.
In a particular variant of this implementation, the peripheral grooves comprise an outer first groove whose opening is directed towards the outside of the insert, an outer second groove whose opening faces along the axis of the insert towards the outside of the vessel, and an inner third groove whose opening faces along the axis of the insert towards the outside of the vessel.
Each of these grooves perform two functions, both mechanical connection between the insert and the wall of the vessel, and deformation of the vessel so as to constitute baffles on the path of any hydrocarbon getting round the insert to escape from the vessel.
The insert can be shaped in such a manner that the vessel has a reentrant corner situated in the inside space of the insert.
The external mouth formed by the insert can be suitable for receiving a pipe that can be fixed to the mouth, e.g. by welding.
The material from which the insert is made is preferably selected to be compatible with that constituting the pipe, so as to enable the pipe to be welded to the insert and form a continuous junction that eliminates any risk of hydrocarbon emission.
In a variant, the insert can have a thread for receiving a separate element such as a cap, which can be put into place by being screwed onto the mouth.
The present invention also provides an insert for use in implementing the above-described method.
The present invention also provides a fuel tank vessel comprising a tubular insert for constituting an external mouth suitable for putting the inside of the vessel into communication with the outside, the vessel comprising at least a first layer forming a hydrocarbon barrier and a second layer that is more permeable to hydrocarbons than the first and that is on the outside of the first layer, and wherein the insert is shaped so as to lengthen the diffusion path for hydrocarbons diffusing along the second layer from the edge of the vessel adjacent to the insert towards the outside of the vessel, and/or to form reductions in thickness in said second layer along the diffusion path.